feat: multi-project workspaces and project window groups (#448)

Workspaces open several local project folders in one IDE session (File › New / Open / Manage Workspace…, or Attach when opening a project, which creates a workspace automatically for a plain project window). Member folders are the Project view roots and take part in indexing, Find in Files, Git, language servers, run configurations and builds; agent tasks choose which member they work in. Window › Merge All Project Windows groups project windows into one window with project tabs (switch, reorder, close, move to a new window), restored on reopen.

Closes #241
Closes #198
Closes #286

src/features/project-tree/ProjectView.test.tsx

@@ -243,3 +243,11 @@ describe("ProjectView", () => {
     expect(rendered).toBeLessThan(80);
   });
 });
+
+it("renders workspace member roots with their own paths, without the settings directory", async () => {
+  fs = new MemoryFs({ "/z/first.txt": "", "/a/second.txt": "", "/workspace/.openidea/workspace.json": "{}" });
+  setProjectFsForTests(fs, "/workspace");
+  initProjectTree("/workspace", "Workspace", ["/z", "/a"]);
+  const { rows } = setup();
+  await waitFor(() => expect(rows()).toEqual(["z/z", "first.txt", "a/a", "second.txt"]));
+});